1. Field of the Invention
This invention relates to the polymerization of polydiorganosiloxanes in the presence of reinforcing filler while the ingredients are maintained as powders.
2. Description of the Prior Art
Silicone rubber has been manufactured by first polymerizing various diorganosiloxane oligomers to higher molecular weight, then reinforcing the resulting polymer by mixing in a filler such as colloidal silica. The dispersion of the reinforcing filler into the polymer requires a great deal of energy, particularly when the polymer has a high enough molecular weight to yield a gum.
Corrin, in U.S. Pat. No. 2,803,617, issued Aug. 20, 1957, claims a process of preparing filler-containing organopolysiloxanes in particulate form. His process is carried out by heating a mixture of a cyclic organosiloxane, a solid filler, and catalytic amounts of ferric halides. After heating, a free flowing powder results which can be milled, catalyzed, and molded in the same manner as conventionally produced silicone rubber compositions. The powdered rubber does not develop "structure" upon storage. Corrin teaches that when potassium hydroxide, a common polymerization catalyst, is used in his process in place of the ferric halide, a gum results rather than a powdered product.
Buchner et al., in British Pat. No. 1,325,654, issued Aug. 8, 1973, disclose a process for manufacture of a mixture of high molecular weight linear organopolysiloxane with an acidic or neutral filler. Their method mixes together, in a mixing and kneading device, a low molecular weight organocyclosiloxane, filler, and anhydrous perfluoroalkane sulfonic acid.